The device of the present invention generally relates to apparatus for protecting electrical equipment from damage or destruction due to the presence of electrical overvoltage surges, commonly referred to as a surge arrester, and, more particularly, to a new and improved device for separating an electrical ground lead wire from a damaged surge arrester, commonly referred to as a disconnector.
B. Description of the Prior Art
A surge or lightning arrester is commonly electrically connected across a comparatively expensive piece of electrical equipment to shunt overvoltage surges, for example, overvoltage surges due to lightning strokes, to ground to thereby protect the piece of electrical equipment from damage or destruction. Occasionally, the surge arrester itself is damaged, for example, by a relatively large overvoltage surge and may then continuously conduct power system follow current subsequent to the overvoltage surge to ground through a ground lead wire attached to the surge arrester. When the damaged surge arrester is equipped with a ground lead wire explosive disconnector, the disconnector is designed to react to the continuous current flow and to explosively separate the ground lead wire from the damaged arrester. Typical prior art ground lead disconnectors are disclosed in U.S. Pat. No. 3,869,650 and in the patents referred to therein.
Typical prior art disconnectors contain an explosive cartridge that, when sufficiently heated, explodes and separates a ground lead wire from a damaged arrester. Some problems with typical prior art disconnectors relate to the control of the rate of heating of the explosive cartridge. One of the problems has been to provide a disconnector which can safely conduct either lightning or surge currents exceeding 100,000 amperes or half cycle follow currents exceeding 500 amperes, yet which will properly disconnect a ground lead wire from a damaged arrester when conducting only twenty amperes for a specified time. Another problem has been to assure that the explosive cartridge charge will forcefully explode to properly disconnect a ground lead wire from a damaged arrester. Under some conditions, the explosive cartridge charge may burn slowly and not forcefully explode to separate the ground lead wire from a damaged arrester. For example, electrical arcing to a single spot on an explosive cartridge metallic casing can, under specific conditions, burn through the casing and ignite the explosive charge before the charge reaches an explosive temperature. As a result, the explosive charge may burn out slowly and fail to separate a ground lead wire from a damaged arrester.
Several U.S. patents disclose disconnectors that are designed to solve one or more of the above problems. For example, U.S. Pat. No. 4,204,238 discloses a disconnector that fully encloses the elongated body of an explosive cartridge 22 and permits arcing only to a refractory metal coating 23 on the top section or head of the explosive cartridge 22. U.S. Pat. No. 3,369,091 discloses a disconnector in which an explosive cartridge 38 is enclosed within a resistance member 46 and is designed to permit arcing to a copper cap 44 fitted to an end of the explosive cartridge 38. U.S. Pat. No. 3,679,938 discloses a disconnector that is designed to improve the reliability of the explosive cartridge by providing for arcing to the shank or body portion of the explosive cartridge proximate to the center of mass of the explosive charge in the cartridge. In a related commercial embodiment of the disconnector disclosed in the above U.S. Pat. No. 3,679,938, a metallic sleeve surrounds the body or shank portion of the explosive cartridge and forms one of the gap electrodes. U.S. Pat. No. 3,710,212 discloses a disconnector in which an explosive cartridge is encased within both a metallic casing 31 and a resistive elastomeric gasket 32 and in which arcing may occur to the casing 31, but may also switch directly to the exposed head 29 of the explosive cartridge. Finally, U.S. Pat. No. 2,824,928 discloses a disconnector that provides for deliberate arc switching wherein arcing initially occurs in a spaced series arc gap 24 and switches progressively along a metallic electrode 22 to eventually contact the exposed head of an explosive cartridge 46 disposed in an aperture 43 of the electrode 22.